It has been demanded to provide organic materials having excellent heat resistance as well as excellent flexibility as materials for electrical apparatuses and electronic equipment with a tendency to larger capacity, smaller-sized weight reduction or higher reliability and higher density in recent years.
Polyaminobismaleimide resins (e.g., polyimide resins composed of N,N'-diphenylmethanebismaleimide and 4,4'-diaminodiphenylmethane described in JP-B-46-23250) (the term "JP-B" as used herein means an examined Japanese patent publication") have been conventionally used in the field of heat-resistant laminated sheets for which high heat resistance, through-hole bonding reliability, dimensional stability, electrical characteristics are required.
However, the cured articles of the above-mentioned resins have problems in that heat resistance, flexibility and dielectric properties are lowered.
In addition, the polyaminobismaleimide resins have poor solubility and are dissolved in only polar organic solvents having a very high boiling point so that they have problems in drying stage and pressing when substrates are impregnated with them to prepare prepregs.
With the development of large-sized computers in particular, the dielectric constant of substrates must be lowered to speed up the signal transfer speed of computers as is clear from the following formula. ##EQU1##
Under the circumstances, polyimide resins and fluoro resins excellent in heat resistance, flexibility and electrical properties are drawing attention.
However, the substrates of the former polyimide resins have a dielectric constant of as high as 4 or higher at a frequency of 1 MHz. The substrates of the latter fluoro resins have a dielectric constant of about 2.5 and are considered to have an appropriate characteristic value. However, the latter have disadvantages in that processability and dimensional stability are inferior, particularly dimensional stability under high-temperature conditions in multi-layering stage during the preparation of multi-layer circuit boards is very inferior.
The present invention has been accomplished with the view of solving the above-described problems associated with prior arts.